Solid oxide fuel cells (hereinafter referred to as “SOFCs”) have high efficiency and require no expensive catalyst such as platinum or the like. SOFCs, however, have a problem in that they operate at high temperatures, i.e., 800° C. to 1,000° C., at which components such as interconnectors or the like deteriorate readily.
To solve the above problem, medium-temperature SOFCs, which operate at temperatures of 600° C. or lower, are expected to be useful. Unfortunately, SOFCs that operate at low operating temperatures have a problem in that they have low efficiency and cannot provide the necessary power generation performance. Accordingly, there is a need for a solid electrolyte that has high efficiency and can provide the necessary power generation performance at low operating temperatures.
Solid electrolytes include oxygen-ion conducting solid electrolytes and proton-conducting solid electrolytes. The use of oxygen-ion conducting solid electrolytes has a problem in that oxygen ions combine with hydrogen on the fuel electrode to produce water, which dilutes the fuel and thus decreases the efficiency.
Proton-conducting solid electrolytes, such as yttrium-doped barium zirconate (hereinafter referred to as “BZY”) or the like, which have low activation energy for carrier transportation and thus provide high proton conductivity at low temperatures, are expected to be useful as an alternative to oxygen-ion conducting solid electrolytes. The use of proton-conducting solid electrolytes also eliminates the above problem with oxygen-ion conducting solid electrolytes.